[mypyc] Don't crash on unreachable statements (#16311)

Skip them instead. This applies to statements after break, continue,
return and raise statements.

It's common to have unreachable statements temporarily while working on
a half-finished change, so generating an error is perhaps not the best
option.

Fixes mypyc/mypyc#1028.

mypyc/irbuild/builder.py

@@ -165,6 +165,9 @@ def __init__(
         self.runtime_args: list[list[RuntimeArg]] = [[]]
         self.function_name_stack: list[str] = []
         self.class_ir_stack: list[ClassIR] = []
+        # Keep track of whether the next statement in a block is reachable
+        # or not, separately for each block nesting level
+        self.block_reachable_stack: list[bool] = [True]
 
         self.current_module = current_module
         self.mapper = mapper
@@ -1302,6 +1305,14 @@ def is_native_attr_ref(self, expr: MemberExpr) -> bool:
             and not obj_rtype.class_ir.get_method(expr.name)
         )
 
+    def mark_block_unreachable(self) -> None:
+        """Mark statements in the innermost block being processed as unreachable.
+
+        This should be called after a statement that unconditionally leaves the
+        block, such as 'break' or 'return'.
+        """
+        self.block_reachable_stack[-1] = False
+
     # Lacks a good type because there wasn't a reasonable type in 3.5 :(
     def catch_errors(self, line: int) -> Any:
         return catch_errors(self.module_path, line)

mypyc/irbuild/statement.py

@@ -118,8 +118,13 @@
 
 def transform_block(builder: IRBuilder, block: Block) -> None:
     if not block.is_unreachable:
+        builder.block_reachable_stack.append(True)
         for stmt in block.body:
             builder.accept(stmt)
+            if not builder.block_reachable_stack[-1]:
+                # The rest of the block is unreachable, so skip it
+                break
+        builder.block_reachable_stack.pop()
     # Raise a RuntimeError if we hit a non-empty unreachable block.
     # Don't complain about empty unreachable blocks, since mypy inserts
     # those after `if MYPY`.

mypyc/irbuild/visitor.py

@@ -194,6 +194,7 @@ def visit_expression_stmt(self, stmt: ExpressionStmt) -> None:
 
     def visit_return_stmt(self, stmt: ReturnStmt) -> None:
         transform_return_stmt(self.builder, stmt)
+        self.builder.mark_block_unreachable()
 
     def visit_assignment_stmt(self, stmt: AssignmentStmt) -> None:
         transform_assignment_stmt(self.builder, stmt)
@@ -212,12 +213,15 @@ def visit_for_stmt(self, stmt: ForStmt) -> None:
 
     def visit_break_stmt(self, stmt: BreakStmt) -> None:
         transform_break_stmt(self.builder, stmt)
+        self.builder.mark_block_unreachable()
 
     def visit_continue_stmt(self, stmt: ContinueStmt) -> None:
         transform_continue_stmt(self.builder, stmt)
+        self.builder.mark_block_unreachable()
 
     def visit_raise_stmt(self, stmt: RaiseStmt) -> None:
         transform_raise_stmt(self.builder, stmt)
+        self.builder.mark_block_unreachable()
 
     def visit_try_stmt(self, stmt: TryStmt) -> None:
         transform_try_stmt(self.builder, stmt)

mypyc/test-data/irbuild-unreachable.test

@@ -1,4 +1,4 @@
-# Test cases for unreachable expressions
+# Test cases for unreachable expressions and statements
 
 [case testUnreachableMemberExpr]
 import sys
@@ -104,3 +104,138 @@ L5:
 L6:
     y = r11
     return 1
+
+[case testUnreachableStatementAfterReturn]
+def f(x: bool) -> int:
+    if x:
+        return 1
+        f(False)
+    return 2
+[out]
+def f(x):
+    x :: bool
+L0:
+    if x goto L1 else goto L2 :: bool
+L1:
+    return 2
+L2:
+    return 4
+
+[case testUnreachableStatementAfterContinue]
+def c() -> bool:
+    return False
+
+def f() -> None:
+    n = True
+    while n:
+        if c():
+            continue
+            if int():
+                f()
+        n = False
+[out]
+def c():
+L0:
+    return 0
+def f():
+    n, r0 :: bool
+L0:
+    n = 1
+L1:
+    if n goto L2 else goto L5 :: bool
+L2:
+    r0 = c()
+    if r0 goto L3 else goto L4 :: bool
+L3:
+    goto L1
+L4:
+    n = 0
+    goto L1
+L5:
+    return 1
+
+[case testUnreachableStatementAfterBreak]
+def c() -> bool:
+    return False
+
+def f() -> None:
+    n = True
+    while n:
+        if c():
+            break
+            if int():
+                f()
+        n = False
+[out]
+def c():
+L0:
+    return 0
+def f():
+    n, r0 :: bool
+L0:
+    n = 1
+L1:
+    if n goto L2 else goto L5 :: bool
+L2:
+    r0 = c()
+    if r0 goto L3 else goto L4 :: bool
+L3:
+    goto L5
+L4:
+    n = 0
+    goto L1
+L5:
+    return 1
+
+[case testUnreachableStatementAfterRaise]
+def f(x: bool) -> int:
+    if x:
+        raise ValueError()
+        print('hello')
+    return 2
+[out]
+def f(x):
+    x :: bool
+    r0 :: object
+    r1 :: str
+    r2, r3 :: object
+L0:
+    if x goto L1 else goto L2 :: bool
+L1:
+    r0 = builtins :: module
+    r1 = 'ValueError'
+    r2 = CPyObject_GetAttr(r0, r1)
+    r3 = PyObject_CallFunctionObjArgs(r2, 0)
+    CPy_Raise(r3)
+    unreachable
+L2:
+    return 4
+
+[case testUnreachableStatementAfterAssertFalse]
+def f(x: bool) -> int:
+    if x:
+        assert False
+        print('hello')
+    return 2
+[out]
+def f(x):
+    x, r0 :: bool
+    r1 :: str
+    r2 :: object
+    r3 :: str
+    r4, r5 :: object
+L0:
+    if x goto L1 else goto L4 :: bool
+L1:
+    if 0 goto L3 else goto L2 :: bool
+L2:
+    r0 = raise AssertionError
+    unreachable
+L3:
+    r1 = 'hello'
+    r2 = builtins :: module
+    r3 = 'print'
+    r4 = CPyObject_GetAttr(r2, r3)
+    r5 = PyObject_CallFunctionObjArgs(r4, r1, 0)
+L4:
+    return 4